In my prior Canadian patent No. 1,153,878, I disclosed a machine and method (collectively a ‘system’) for constructing the side wall of a large cylindrical steel tank from a coil of steel strip. In general, this system involved:                building a circular steel floor for the tank on the ground, by welding together appropriately shaped flat plates;        building a circular roof on the floor by welding together a circumferential rolled channel rim, radial struts extending between a central sleeve and the rim, and plates supported by the struts;        raising the roof into the air with a circumferential array of lifting jacks and a central column extending through the sleeve;        tacking and then continuously welding a length of strip to the circular edge of the roof rim to establish the first course and initiate the downwardly extending side wall;        suspending the roof and side wall (now collectively referred to as “the tank”) up in the air with the jacks holding the bottom rim of the wall;        positioning a steerable, powered, pneumatic wheel-supported machine on the tank floor adjacent the floor's side edge, the machine carrying a rotatable coil from which strip could be pulled or unwound;        advancing the machine along the circular edge of the floor while paying out strip from the coil. The paid-out strip was partly straightened by passing it through crush rolls to change its curvature to closely approximate that of the tank wall. The straightened strip was then guided and manipulated so that its top edge was aligned with the bottom edge of the first course in slightly gapped relationship at a “fit up and weld point” (hereinafter referred to as the “tack point”). A tack weld was then applied to join the wall and strip at this point. This process was repeated until a substantially complete second ring or course of strip had been tack welded to the tank wall. The end of the welded strip was then cut from the coil material. A continuous butt weld was thereafter applied along the horizontal wall/strip joint line, to complete joining the new ring of strip to the wall; and        the structure was then further raised and the process repeated to add further courses to the wall of the developing tank. In this way the tank side wall was fully constructed and then welded to the floor to complete the structure.        
The '878 machine was operated over a number of years in the construction of large oil storage tanks, using this method of unwinding coiled steel strip, partly straightening it, manipulating it into a gapped relationship with the tank wall at the tack point, and then tacking and butt welding it to the bottom rim of the suspended elevated tank wall.
However it was apparent throughout that there was need for a machine better able to manipulate and position the steel strip and the tank wall into optimum alignment, curvature, verticality or ‘plumbness’ and gapping, all at the tack point. This was not an easy problem to address, given:                that the strip approaches the tack point from the inside and at an angle;        that there are several factors which affect the outcome, such as the flabbiness and flexibility of the tank wall and strip, the unevenness of the floor on which the machine travels, changes in weight on the pneumatic tire-supported machine such as occur when steel is removed from the coil, and sagging of the side wall when the support of a jack, in the way of the machine, is removed; and        that the gap width or spacing at the tack point has to be finely and accurately controlled with extreme precision. More particularly, one needs to be able to control the width of the gap, at the tack point, with an accuracy of about 0.01 inches (in the case of a 100′ diameter tank having 2″ tack welds at 8″ centers). This is required because, if the desired gap at the cooled weld is to be 1/16″, one needs to allow 0.01″ extra for weld shrinkage (so the gap spacing at the tack point, prior to welding, should be 1/16″+0.01″).        
The present invention is the result of a far-reaching re-design of the machine and the steps practiced, to address these issues.